Refactoring skolems for sets (#3381)

Refactoring skolems for sets (#3381)

This refactors skolems introduced in the theory of sets. This is analogous to how skolems are treated for the theory of strings.

A key change that this commit enables is to identify "variable" sets based on those that weren't introduced by the SkolemCache (instead of via a check that their kind is `VARIABLE`, which is done currently and is error prone).